Distribution of macrobenthic animals on the flat bottom in Obama Bay.

Abstract

The distribution of smaller macrobenthos (>1mm, >1g wet weight) was investigated by collecting samples of bottom sediment at 38 stations in Obama Bay, Japan in June, 1977. Quantitative duplicate samples were obtained at each station with a small type Smith-McIntyre bottom sampler (0.05m2) and were washed through a 1-mm mesh screen. Ampelisca brevicornis (amphipod), Polydora flava orientalis (polychaete), Veremolpa micra and Theora lubrica (bivalves) were the most abundant species present. The bay is divided into eight areas (A-H), on the basis of the similarity of the species composition of 24 commonly occurring species. The faunal characteristics of each area are described in detail. A comparison with previous work carried out more than 30 years ago revealed considerable increases of the numerical density and biomass during the period between the two surveys. These quantitative differences, however, might be due to the difference of the sampling methods adopted, including the sampling gears used. Comparison also suggested that some faunal changes have occurred during the past thirty years. The drastic disappearance of a Ditrupa dominant area from the inner part of the bay is interpreted in relation to the enlargement of the muddy bottom area in this locality.